(a) Field of the Invention
The present invention relates to a driving and controlling apparatus for controlling the output torque of a synchronous motor using permanent magnets as its field system (hereafter referred to as PM motor) so as to attain a reference torque by controlling the vector of a motor current. Particularly, it relates to the control of a PM motor which can be suitably mounted on an electric vehicle and can execute field weakening control.
(b) Description of the Prior Art
A PM motor is a synchronous motor using permanent magnets as its field system. Therefore, it is characterized by having a large magnetomotive force per unit volume. This characteristic is effective for increasing the output of a motor and reducing the size of a motor. Particularly, the PM motor is preferable as the drive motor of an electric vehicle.
FIG. 20 shows an equivalent circuit per phase of a PM motor. In FIG. 20, the magnetomotive force of permanent magnets used as a field system, that is, the main magnetic flux of the PM motor, is shown by the symbol E.sub.o. When assuming the axial angular velocity of the PM motor as .omega. (=2.pi.N, N: rotational speed of the motor), the counter electromotive force generated during operation of the PM motor can be expressed as .omega.E.sub.o. When the PM motor is driven by converting the discharge output of a battery into alternating current by an inverter and using the AC power, the terminal voltage V of the PM motor is obtained by multiplying a battery voltage V.sub.dc by the voltage conversion ratio of the inverter. In FIG. 20, symbol R represents the primary resistance per phase of the PM motor, L represents the inductance per phase, and I represents the primary current (phase current).
The motor current I of the PM motor can be decomposed into the vector components, i.e., the field current I.sub.d and the torque current I.sub.q. The field current I.sub.d is a component for generating a field flux in the PM motor and the torque current I.sub.q is a component for generating torque by intersecting with the field flux of permanent magnets, that is, the main magnetic flux E.sub.o. When the PM motor is provided with saliency, the field current I.sub.d also generates torque. The torque generated when the torque current I.sub.q intersects with the main magnetic flux E.sub.o is known as magnetic torque and the torque caused by the saliency is known as reluctance torque. The following equation (1) is an expression showing the torque T of the PM motor, In which the first term at the right side shows magnetic torque and the second term shows reluctance torque. Therefore, the second term does not appear when the PM motor is a non-salient-pole motor. EQU T=E.sub.o +(L.sub.d -L.sub.q)I.sub.d I.sub.q ( 1)
L.sub.d and L.sub.q in the equation (1) are called d-axis inductance and q-axis inductance respectively, which are the d-axis component and q-axis component of the inductance L of the PM motor respectively. The terminal voltage V of the PM motor can also be decomposed into the d-axis component and q-axis component. The d-axis component and q-axis component of the terminal voltage V, that is, d-axis voltage V.sub.d and q-axis voltage V.sub.q, can be expressed as shown below by using the field current I.sub.d, torque current I.sub.q, d-axis inductance L.sub.d, and q-axis inductance L.sub.q when the number of pole pairs is 1. EQU V.sub.d =RI.sub.d +j.omega.L.sub.q L.sub.q EQU V.sub.q =j.omega.L.sub.d I.sub.d +RI.sub.q +.omega.E.sub.o ( 2)
j: An Imaginary component PA1 (a) current control means for controlling the primary current of the PM motor in accordance with a reference current; and PA1 (b) current condition determination means for minimizing the reference current in accordance with the voltage of a battery under a predetermined condition and at least partially cancelling the field flux generated by the permanent magnets with the field flux generated by the field current component so that the counter electromotive force of the PM motor does not exceed the voltage of the battery and so that the field flux generated by the field current component has an intensity corresponding to the voltage of the battery. PA1 (a) current control means for controlling a primary current of the PM motor in accordance with a reference current; and PA1 (b) current condition determination means having; PA1 (b1) counter electromotive force judgment means for judging whether or not the counter electromotive force of a synchronous motor exceeds the voltage of the battery in accordance with the output state of the PM motor; PA1 (b2) means for determining a reference current so that the field flux generated by the field current component is fixed to 0 or a specific value, when it is judged that the counter electromotive force does not exceed the voltage of the battery; and PA1 (b3) means for minimizing the reference current in accordance with the voltage of the battery so that the counter electromotive force of the PM motor does not exceed the voltage of the battery and so that the field flux generated by the field current component has an intensity corresponding to the voltage of the battery, and at least partially cancelling the field flux generated by permanent magnets with the field flux generated by the field current component, when it is judged that the counter electromotive force exceeds the voltage of the battery. PA1 (a) judging whether or not the counter electromotive force of the PM motor exceeds the voltage of the battery in accordance with the output state of the PM motor; PA1 (b) determining a reference current so that the field flux generated by the field current component is fixed to 0 or a specific value, when it is judged that the counter electromotive force does not exceed the voltage of the battery; PA1 (c) minimizing the reference current in accordance with the voltage of the battery so that the counter electromotive force of the PM motor does not exceed the voltage of the battery and so that the field flux generated by the field current component has an intensity corresponding to the battery voltage, when it is judged that the counter electromotive force exceeds the voltage of the battery; and PA1 (d) controlling a primary current of the PM motor in accordance with the reference current to at least partially cancel the field flux generated by permanent magnets with the field flux generated by the field current component. PA1 (a) current control means for controlling the primary current of the PM motor in accordance with a reference current; and PA1 (b) current condition determination means having: PA1 (a) judging whether or not either of a reference torque or an output state of the PM motor change by as much as a predetermined value or more; PA1 (b) determining an initial reference current in accordance with the reference torque, the output state of the PM motor and a reference minimum voltage on the assumption that the voltage of the battery is the reference minimum voltage, when either of the reference torque or the output state of the PM motor change by as much as a predetermined value or more; PA1 (c) controlling the primary current of the PM motor in accordance with the initial reference current; PA1 (d) determining the reference current in accordance with a converged battery voltage so that the counter electromotive force of the PM motor does not exceed the voltage of the battery and so that the field flux generated by the field current component has an intensity corresponding to the voltage of the battery, when the voltage of the battery is converged to a constant value after the primary current of the PM motor has been controlled in accordance with the initial reference current; and PA1 (e) controlling the primary current of the PM motor in accordance with the reference current to at least partially cancel the field flux generated by permanent magnets with the field flux generated by the field current component. PA1 (a) current control means for controlling a primary current of the PM motor in accordance with reference current; where an optimum current depends on the voltage of the battery when the output state of the synchronous motor is in a predetermined region and does not depend on the voltage of the battery when the output state of the synchronous motor is not in the predetermined region, the optimum current representative of the reference current where the reference torque is realized as the torque of the synchronous motor and the efficiency of the synchronous motor substantially becomes a maximum efficiency; and PA1 (b) current condition determination means having: PA1 (b1) means for determining an initial reference current in accordance with the reference torque, the output state of the PM-motor and a reference minimum voltage on the assumption that the voltage of the battery is the reference minimum voltage, when either of the reference torque or the output state of the PM-motor change by as much as a predetermined value or more, and giving the determined initial reference current as the reference current to the current control means; PA1 (a) judging whether or not either of a reference torque or an output state of the PM motor change by as much as a predetermined value or more; PA1 (b) determining an initial reference current in accordance with the reference torque, the output state of the PM-motor and a reference minimum voltage on the assumption that the voltage of the battery is the reference minimum voltage, when either of the reference torque or the output state of the PM-motor change by as much as a predetermined value or more; PA1 (c) controlling the primary current of the PM motor in accordance with the initial reference current; PA1 (d) obtaining a first optimum reference current by referring to a relationship between the voltage of the battery, an optimum current and the output state of the PM-motor, by the converged battery voltage and PM-motor output state in a region where a value of the optimum current depends on the voltage of the battery, when the voltage of the battery is converged to a constant value after a primary current of the PM motor has been controlled in accordance with the initial reference current; wherein an optimum current depends on the voltage of the battery when the output state of the synchronous motor is in a predetermined region and does not depend on the voltage of the battery when the output state of the synchronous motor is not in the predetermined region, the optimum current representative of the reference current where the reference torque is realized as the torque of the synchronous motor and the efficiency of the synchronous motor substantially becomes a maximum efficiency; PA1 (e) determining a second optimum reference current from the optimum current in a region where the optimum current does not depend on the voltage of the battery; PA1 (f) selecting either of the first or second optimum reference currents so that the counter electromotive force of the PM motor does not exceed the voltage of the battery and so that the field flux generated by the field current component has an intensity corresponding to the voltage of the battery, on the basis of a relationship between a magnitude of the first optimum reference current and that of the second optimum reference current; PA1 (g) determining the selected optimum current as the reference current; and PA1 (h) controlling the primary current of the PM motor in accordance with the reference current to at least partially cancel the field flux generated by permanent magnets with the field flux generated by the field current component.
Moreover, the following relationship exists between d-axis voltage V.sub.d and q-axis voltage V.sub.q. EQU .vertline.V.vertline..sup.2 =.vertline.V.sub.d .vertline..sup.2 +.vertline.V.sub.q .vertline..sup.2 ( 3)
In general, because the primary resistance R of the PM motor is much smaller than the inductance, it is possible to ignore the resistance R. When the primary resistance R is ignored, the equation (2) is expressed as shown below. FIG. 21 shows a vector diagram of the PM motor drawn in accordance with the equation (4). EQU V.sub.d =j.omega.L.sub.q I.sub.q EQU V.sub.q =j.omega.L.sub.d I.sub.d +.omega.E.sub.o EQU V=.omega.E.sub.o +j.omega.L.sub.q I.sub.q +j.omega.L.sub.d I.sub.d( 4)
To control the PM motor having the above characteristics, it is preferable to control the vector of the motor current I. For example, it is preferable to decompose the motor current I into the field current component I.sub.d and the torque current component I.sub.q and control the absolute value of each component. To control the vector of the motor current I of the PM motor, the controller receives a command for a vector to be outputted from the PM motor, that is, a reference torque T.sup..revreaction. from a supervisor controller. The controller receiving the reference torque T.sup..revreaction. determines the field current I.sub.d and the torque current I.sup..revreaction. which are control targets so that the reference torque T.sup..revreaction. is realized. In this case, it is not necessary to excite the PM motor with the field current I.sub.d, because permanent magnets generate the main magnetic flux. Therefore, the field current I.sub.d is normally controlled to 0 or some other specified value. When the field current I.sub.d is kept constant, it is possible to control the torque T with the torque current I.sub.q as shown by the equation (1). Therefore, the reference torque T.sup..revreaction. is exclusively used to determine the torque current I.sub.q. The controller outputs the determined field current and torque current I.sub.q to a motor controller as a reference field current I.sub.d.sup..revreaction. and a reference torque current I.sub.q respectively. The motor controller controls a power converter such as an inverter so that the field current I.sub.d and the torque current I.sub.d.sup..revreaction. corresponding to the reference field current I.sub.d.sup..revreaction. and the reference torque current I.sub.q.sup..revreaction. respectively flow through the PM motor. Though the motor current I can be decomposed into the vector components of the field current I can be and the torque current I.sub.q as shown above, it is also possible to decompose the motor current I into an absolute value component .vertline.I.vertline. and a phase component argI.
To control the vector of the current I of the PM motor, it is preferable to perform the field weakening control at the same time. The field weakening control is defined as the control for weakening a field flux when the PM motor rotates at a high speed.
As shown in the equation (4) and FIG. 21, when the rotational speed N of the PM motor, namely, the axial angular velocity .omega. of the PM motor increases, the counter electromotive force .omega.E.sub.o increases in proportion to the increase of the axial angular velocity .omega. and the terminal voltage V of the PM motor increases with the increase of the counter electromotive force .omega.E.sub.o. When the terminal voltage V of the PM motor exceeds a value corresponding to the battery voltage V.sub.dc, a voltage corresponding to the difference between the terminal voltage V and the battery voltage V.sub.dc is applied to an inverter provided between the PM motor or the power source. This voltage causes the inverter or the like to be damaged. Hereafter, the rotational speed N of the PM motor when the terminal voltage V equals a value corresponding to the battery voltage V.sub.dc is referred to as a base rotational speed N.sub.B. The region of rotational speed equal to or higher than the base rotational speed N.sub.B is referred to as a high rotation region, the region of rotational speed equal to or lower than the base rotational speed N.sub.B is referred to as a low rotation region, and the region of rotational speed close to the base rotational speed N.sub.B is referred to as a medium rotation region.
In the PM motor, a field flux is mainly generated by permanent magnets. However, when the vector of the motor current I is controlled as described above, it is possible to generate a field flux having a necessary intensity by utilizing the field current I.sub.d, and also to generate a field flux with an intensity which partially cancels the field flux (main magnetic flux) E.sub.o generated by the permanent magnets. The field weakening control can specifically be executed as the control for generating a field flux with an intensity which partially cancels the main magnetic flux E.sub.o, by controlling the field current I.sub.d to a negative value. When this type of control is performed, the absolute value of the terminal voltage V decreases as shown by the equation (4) and FIG. 21. Therefore, when performing the field weakening control by aiming at the field current I.sub.d with a proper intensity, it is possible to control the terminal voltage V in the high rotation region to the battery voltage V.sub.dc or lower. The field current I.sub.d when the field weakening control is executed is referred to as a field weakening current.
To execute the field weakening control, the value of the field weakening current I.sub.d is normally determined in accordance with the rotational speed N of the PM motor and the requested output torque (=reference torque T.sup..revreaction.) by assuming that the battery voltage V.sub.dc is constant. For example, as shown in FIG. 22, the field current I.sub.d is controlled to 0 in the low rotation region. As the rotational speed N increases exceeding the base rotational speed N.sub.B, the effective amplitude of the field weakening current I.sub.d is varied from -30 A to -50 A, to -100 A , . . . in that order.
The field weakening control is disclosed in "Variable Speed Drive System of Permanent Magnet Synchronous Motors with Flux-weakening Control", Keita HATANAKA et al., at the 1991 General Conference No. 74 of the Industrial Application Division of The Institute of Electrical Engineers of Japan, pp 310-315.
However, because the actual battery voltage V.sub.dc greatly depends on its state of charge (SOC) and load state, the field weakening current I.sub.d may become too large or too small. This causes a problem that the efficiency lowers or the reference torque T.sup..revreaction. is not accurately realized.
In this case, a value obtained by converting the battery voltage V.sub.dc, assumed to determine the field weakening current I.sub.d, into a value comparable with the terminal voltage V is referred to as an allowable power-source voltage V.sub.B. When the allowable power-source voltage V.sub.B is higher than the actual battery voltage V.sub.dc, the field weakening current I.sub.d with an absolute value excessively larger than reference torque T.sup..revreaction. flows. In other words, the absolute value of the field current I.sub.d increases, which is a current component not contributing to generation of torque. This means that the efficiency lowers. Conversely, when the allowable power-source voltage V.sub.B is lower than the actual battery voltage V.sub.dc, it is impossible to provide sufficient field weakening current I.sub.d to control the terminal voltage V to the allowable power-source voltage V.sub.B or lower, or sufficient torque current I.sub.q to realize the reference torque T.sup..revreaction..